Some general rules of thumb are to have tweeters at ear level and then experiment from there. As for the center, you will probably need to angle it up or down depending on if it is above or below the TV. I used to have a VP150 above my projector screen years ago and it was OK, but when I moved it to below, it somehow sounded better. Both were angled (down when above the screen, and up when below).
My other though, and this is from a personal revelation in my current theater mainly... Smaller spaces really are "reflection chambers." You have almost every surface close enough that sound just bounces around the room like crazy. Instead of just getting primary reflections, a smaller room will also have strong secondary reflections as well. If you can do something, even temporarily, to absorb some of those reflections in the room, I believe that the M80s would really start to sing again...
My previous room was 10' x 15' and I had similar imaging issues. Before we moved from that house, I did an experiment with a (twin sized) mattress. I just put up on its side along the one wall. Wow. Big sound improvement. And then I tore down the theater getting ready to move. It was at that point that I knew that I wanted sound treatments in my next (current) theater.
With so many reflections, you have sounds canceling themselves out as they fly around the room.
So either find something temporary to do some absorption, or just live with the sub-quality sound for a while until you have a more permanent home for them.
